Magnetic recording medium having characterized magnetic layer

ABSTRACT

The magnetic recording medium includes a non-magnetic support and a magnetic layer which contains ferromagnetic powder and a binder, in which the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive, Int (110)/Int (114) of a crystal structure of the hexagonal ferrite, determined by performing X-ray diffraction analysis on the magnetic layer by using an In-Plane method, to a peak intensity of a diffraction peak of (114) plane of the crystal structure is equal to or higher than 0.5 and equal to or lower than 4.0, a squareness ratio of the magnetic recording medium in a vertical direction is equal to or higher than 0.65 and equal to or lower than 1.00, and a logarithmic decrement obtained by performing a pendulum viscoelasticity test on a surface of the magnetic layer is equal to or lower than 0.050.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2017-140016 filed on Jul. 19, 2017 and Japanese PatentApplication No. 2018-131332 filed on Jul. 11, 2018. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic recording medium.

2. Description of the Related Art

Generally, either or both of the recording of information on a magneticrecording medium and the reproduction of information performed bycausing a magnetic head (hereinafter, simply described as “head” aswell) to contact and slide on a surface of the magnetic recording medium(a surface of a magnetic layer).

In order to continuously or intermittently repeat the reproduction ofthe information recorded on the magnetic recording medium, the head iscaused to repeatedly slide on the surface of the magnetic layer(repeated sliding). For improving the reliability of the magneticrecording medium as a recording medium for data storage, it is desirableto inhibit the deterioration of electromagnetic conversioncharacteristics during the repeated sliding. This is because a magneticrecording medium in which the electromagnetic conversion characteristicsthereof hardly deteriorate during the repeated sliding can keepexhibiting excellent electromagnetic conversion characteristics eventhough the reproduction is continuously or intermittently repeated.

Examples of causes of the deterioration of electromagnetic conversioncharacteristics during the repeated sliding include the occurrence of aphenomenon (referred to as “spacing loss”) in which a distance betweenthe surface of the magnetic layer and the head increases. Examples ofcauses of the spacing loss include a phenomenon in which whilereproduction is being repeated and the head is continuously sliding onthe surface of the magnetic layer, foreign substances derived from themagnetic recording medium are attached to the head. In the related art,as a countermeasure for the head attachment occurring as above, anabrasive has been added to the magnetic layer such that the surface ofthe magnetic layer performs a function of removing the head attachment(for example, see JP2005-243162A).

SUMMARY OF THE INVENTION

It is preferable to add an abrasive to the magnetic layer, because thenit is possible to inhibit the deterioration of the electromagneticconversion characteristics resulting from the spacing loss that occursdue to the head attachment. Incidentally, in a case where thedeterioration of the electromagnetic conversion characteristics can besuppressed to a level that is higher than the level achieved by theaddition of an abrasive to the magnetic layer as in the related art, itis possible to further improve the reliability of the magnetic recordingmedium as a recording medium for data storage.

The present invention is based on the above circumstances, and an aspectof the oresent invention provides for a magnetic recording medium inwhich the electromagnetic conversion characteristics thereof hardlydeteriorate even though a head repeatedly slides on a surface of amagnetic layer.

An aspect of the present invention is a magnetic recording mediumcomprising a non-magnetic support and a magnetic layer which is providedon the support and contains ferromagnetic powder and a binder, in whichthe ferromagnetic powder is ferromagnetic hexagonal ferrite powder, themagnetic layer contains an abrasive, an intensity ratio (Int (110)/Int(114)) (hereinafter, described as “XRD (X-ray diffraction) intensityratio” as well) of a peak intensity Int (110) of a diffraction peak of(110) plane of a crystal structure of the hexagonal ferrite, determinedby performing X-ray diffraction analysis on the magnetic layer by usingan In-Plane method, to a peak intensity Int (114) of a diffraction peakof (114) plane of the crystal structure is equal to or higher than 0.5and equal to or lower than 4.0, a squareness ratio of the magneticrecording medium in a vertical direction is equal to or higher than 0.65and equal to or lower than 1.00, and a logarithmic decrement obtained byperforming a pendulum viscoelasticity test on a surface of the magneticlayer (hereinafter, described as “logarithmic decrement of the magneticlayer surface” or simply described as “logarithmic decrement” as well)is equal to or lower than 0.050.

In one aspect, the squareness ratio in a vertical direction may be equalto or higher than 0.65 and equal to or lower than 0.90.

In one aspect, the logarithmic decrement of the magnetic layer surfacemay be equal to or higher than 0.010 and equal to or lower than 0.050.

In one aspect, the magnetic recording medium may further comprise anon-magnetic layer containing non-magnetic powder and a binder betweenthe non-magnetic support and the magnetic layer.

In one aspect, the magnetic recording medium may further comprise a backcoating layer containing non-magnetic powder and a binder on a surface,which is opposite to a surface provided with the magnetic layer, of thenon-magnetic support.

In one aspect, the magnetic recording medium may be a magnetic tape.

According to an aspect of the present invention, it is possible toprovide a magnetic recording medium in which the electromagneticconversion characteristics thereof hardly deteriorate even though a headis caused to repeatedly slide on a surface of a magnetic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for illustrating a method for measuring a logarithmicdecrement.

FIG. 2 is a view for illustrating the method for measuring a logarithmicdecrement.

FIG. 3 is a view for illustrating the method for measuring a logarithmicdecrement.

FIG. 4 shows an example (schematic process chart) of a specific aspectof a process for manufacturing a magnetic tape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aspect of the present invention is a magnetic recording mediumincluding a non-magnetic support and a magnetic layer which is providedon the support and contains ferromagnetic powder and a binder, in whichthe ferromagnetic powder is ferromagnetic hexagonal ferrite powder, themagnetic layer contains an abrasive, an intensity ratio (Int (110)/Int(114)) of a peak intensity Int (110) of a diffraction peak of (110)plane of a crystal structure of the hexagonal ferrite, determined byperforming X-ray diffraction analysis on the magnetic layer by using anIn-Plane method, to a peak intensity Int (114) of a diffraction peak of(114) plane of the crystal structure is equal to or higher than 0.5 andequal to or lower than 4.0, a squareness ratio of the magnetic recordingmedium in a vertical direction is equal to or higher than 0.65 and equalto or lower than 1.00, and a logarithmic decrement obtained byperforming a pendulum viscoelasticity test on a surface of the magneticlayer is equal to or lower than 0.050.

In the present invention and the present specification, “surface of themagnetic layer” refers to a surface of the magnetic recording medium onthe magnetic layer side. Furthermore, in the present invention and thepresent specification, “ferromagnetic hexagonal ferrite powder” refersto an aggregate of a plurality of ferromagnetic hexagonal ferriteparticles. The ferromagnetic hexagonal ferrite particles areferromagnetic particles having a hexagonal ferrite crystal structure.Hereinafter, the particles constituting the ferromagnetic hexagonalferrite powder (ferromagnetic hexagonal ferrite particles) will bedescribed as “hexagonal ferrite particles” or simply as “particles” aswell. “Aggregate” is not limited to an aspect in which the particlesconstituting the aggregate directly contact each other, and alsoincludes an aspect in which a binder, an additive, or the like isinterposed between the particles. The same points as described abovewill also be applied to various powders such as non-magnetic powder inthe present invention and the present specification.

In the present invention and the present specification, unless otherwisespecified, the description relating to a direction and an angle (forexample, “vertical”, “orthogonal”, or “parallel”) includes a margin oferror accepted in the technical field to which the present inventionbelongs. For example, the aforementioned margin of error means a rangeless than a precise angle ±10°. The margin of error is preferably withina precise angle ±5°, and more preferably within a precise angle ±3°.

Regarding the aforementioned magnetic recording medium, the inventors ofthe present invention made assumptions as below.

The magnetic layer of the magnetic recording medium contains anabrasive. The addition of the abrasive to the magnetic layer enables thesurface of the magnetic layer to perform a function of removing the headattachment. However, it is considered that in a case where the abrasivepresent on the surface of the magnetic layer and/or in the vicinity ofthe surface of the magnetic layer fails to appropriately permeate theinside of the magnetic layer by the force applied thereto from the headwhen the head is sliding on the surface of the magnetic layer, the headwill be scraped by contacting the abrasive protruding from the surfaceof the magnetic layer (head scraping). It is considered that in a casewhere the head scraping that occurs as above can be inhibited, it ispossible to further inhibit the deterioration of the electromagneticconversion characteristics caused by the spacing loss.

Regarding the aforementioned point, the inventors of the presentinvention assume that in the ferromagnetic hexagonal ferrite powdercontained in the magnetic layer include particles (hereinafter, referredto as “former particles”) which exert an influence on the degree ofpermeation of the abrasive by supporting the abrasive pushed into theinside of the magnetic layer and particles (hereinafter, referred to as“latter particles”) which are considered not to exert such an influenceor to exert such an influence to a small extent. It is considered thatthe latter particles are fine particles resulting from partial chippingof particles due to the dispersion treatment performed at the time ofpreparing a composition for forming a magnetic layer, for example. Theinventors of the present invention also assume that the more the fineparticles contained in the magnetic layer, the further the hardness ofthe magnetic layer decreases, although the reason is unclear. In a casewhere the hardness of the magnetic layer decreases, the surface of themagnetic layer is scraped when the head slides on the surface of themagnetic layer (magnetic layer scraping), the foreign substancesoccurring due to the scraping are interposed between the surface of themagnetic layer and the head, and as a result, spacing loss occurs.

The inventors of the present invention consider that in theferromagnetic hexagonal ferrite powder present in the magnetic layer,the former particles are particles resulting in a diffraction peak inX-ray diffraction analysis using an In-Plane method, and the latterparticles do not result in a diffraction peak or exert a small influenceon a diffraction peak because they are fine. Therefore, the inventors ofthe present invention assume that based on the intensity of thediffraction peak determined by X-ray diffraction analysis performed onthe magnetic layer by using the In-Plane method, the way the particles,which support the abrasive pushed into the inside of the magnetic layerand exert an influence on the degree of permeation of the abrasive, arepresent in the magnetic layer can be controlled, and as a result, thedegree of permeation of the abrasive can be controlled. The inventors ofthe present invention consider that the XRD intensity ratio, which willbe specifically described later, is a parameter relating to theaforementioned point.

Meanwhile, the squareness ratio in a vertical direction is a ratio ofremnant magnetization to saturation magnetization measured in adirection perpendicular to the surface of the magnetic layer. Thesmaller the remnant magnetization, the lower the ratio. Presumably, itis difficult for the latter particles to retain magnetization becausethey are fine. Therefore, presumably, as the amount of the latterparticles contained in the magnetic layer increases, the squarenessratio in a vertical direction tends to be reduced. Accordingly, theinventors of the present invention consider that the squareness ratio ina vertical direction can be a parameter of the amount of the fineparticles (the latter particles described above) present in the magneticlayer. The inventors of the present invention consider that as theamount of such fine particles contained in the magnetic layer increases,the hardness of the magnetic layer may decrease, and accordingly, thesurface of the magnetic layer may be scraped when the head slides on thesurface of the magnetic layer, the foreign substances that occur due tothe scraping may be interposed between the surface of the magnetic layerand the head, and hence the spacing loss strongly tends to occur.

The inventors of the present invention consider that in theaforementioned magnetic recording medium, each of the XRD intensityratio and the squareness ratio in a vertical direction is in theaforementioned range, and this makes a contribution to the inhibition ofthe deterioration of the electromagnetic conversion characteristicsduring the repeated sliding. According to the inventors of the presentinvention, presumably, this is because the control of the XRD intensityratio mainly makes it possible to inhibit the head scraping, and thecontrol of the squareness ratio in a vertical direction mainly makes itpossible to inhibit the magnetic layer scraping.

The inventors of the present invention consider that, in the magneticrecording medium, an aspect in which the logarithmic decrement obtainedby performing a pendulum viscoelasticity test on a surface of themagnetic layer is equal to or lower than 0.050 makes a contribution tothe further inhibition of the deterioration of electromagneticconversion characteristics during the repeated sliding. This point willbe further described below.

In the present invention and the present specification, the logarithmicdecrement of the magnetic layer surface is a value obtained by thefollowing method.

FIGS. 1 to 3 are views for illustrating a method for measuring alogarithmic decrement. Hereinafter, the method for measuring alogarithmic decrement will be described with reference to the drawings.Here, the aspect shown in the drawings is merely an example, and thepresent invention is not limited thereto.

A portion of the magnetic tape which is a measurement target (ameasurement sample) 100 is placed on a substrate 103 so that ameasurement surface (surface of the back coating layer) faces upwardsand the measurement sample 100 is fixed by, for example, fixing tapes105 in a state where obvious wrinkles which can be visually confirmedare not generated, in a sample stage 101 in a pendulum viscoelasticitytester.

A pendulum-attached round-bar type cylinder edge 104 is loaded on themeasurement surface of the measurement sample 100 so that a long axisdirection of the cylinder edge becomes parallel to a longitudinaldirection of the measurement sample 100. An example of a state in whichthe pendulum-attached round-bar type cylinder edge 104 is loaded on themeasurement surface of the measurement sample 100 as described above(state seen from the top) is shown in FIG. 1. In the aspect shown inFIG. 1, a constitution is illustrated in which a holder and temperaturesensor 102 is installed such that the surface temperature of thesubstrate 103 can be monitored. However, this constitution is notessential. The longitudinal direction of the sample 100 for measurementis a direction indicated by the arrow in the aspect shown in FIG. 1, andrefers to the longitudinal direction in the magnetic recording mediumfrom which the sample for measurement is cut out. Furthermore, as apendulum 107 (see FIG. 2), a pendulum made of a material (for example, ametal, an alloy, or the like) having a property of being adsorbed onto amagnet is used.

The surface temperature of the substrate 103, on which the sample 100for measurement is placed, is increased to 80° C. at a heating rate ofequal to or lower than 5° C./min (the heating rate may be arbitrarilyset as long as it is equal to or lower than 5° C./min), and the pendulum107 is desorbed from a magnet 106 are detached from each other such thatthe pendular movement is initiated (inducing initial oscillation). FIG.2 shows an example of a state where the pendulum 107 is performingpendular movement (state seen sideways). In the aspect shown in FIG. 2,in the pendulum viscoelasticity tester, the electricity conducted to themagnet (electromagnet) 106 disposed below the sample stage is cut off(the switch is turned off) such that the pendulum is desorbed from themagnet and starts to perform pendular movement, and then electricity isconducted again to the electromagnet (the switch is turned on) such thatthe pendulum 107 is adsorbed onto the magnet 106 and stops the pendularmovement. As shown in FIG. 2, while performing pendular movement, thependulum 107 repeatedly swings. While the pendulum is repeatedlyswinging, the displacement of the pendulum is monitored using adisplacement sensor 108. From the obtained result, a displacement-timecurve plotted by taking the displacement as the ordinate and the elapsedtime as the abscissa is obtained. FIG. 3 shows an example of thedisplacement-time curve. FIG. 3 schematically shows how the state of thependulum 107 corresponds to the displacement-time curve. At a regularmeasurement interval, the pendular movement is stopped (adsorption) andresumed repeatedly for 10 minutes or longer (the time may be arbitrarilyset as long as it is equal to or longer than 10 minutes). By using adisplacement-time curve obtained at the measurement interval after 10minutes or longer, a logarithmic decrement Δ (without a unit) isobtained from the following equation, and the value is taken as alogarithmic decrement of the surface of a magnetic layer of a magnetictape. The pendulum is allowed to be adsorbed onto the magnet for 1second or longer (the time may be arbitrarily set as long as it is equalto or longer than 1 second) whenever it is adsorbed onto the magnet, andthe interval from the stop of the adsorption to the initiation of thenext adsorption is set to be 6 seconds or longer (the time may bearbitrarily set as long as it is equal to or longer than 6 seconds). Themeasurement interval is a time interval from the initiation ofadsorption to the next initiation of adsorption. The humidity of theenvironment in which the pendular movement is performed may be arelative humidity that is arbitrarily set as long as it is a relativehumidity within a range of 40% to 70%. Temperature of an environment inwhich the pendulum movement is performed, may be random temperature, aslong as the temperature is 20° C. to 30° C.

$\Delta = \frac{{\ln\left( \frac{A_{1}}{A_{2}} \right)} + {\ln\left( \frac{A_{2}}{A_{3}} \right)} + {\ldots\mspace{14mu}{\ln\left( \frac{A_{n}}{A_{n + 1}} \right)}}}{n}$

In the displacement-time curve, the interval from when the displacementbecomes minimum and to when the displacement becomes minimum again istaken as one period of a wave. n represents the number of waves includedin the displacement-time curve at the measurement interval, and Anrepresents a difference between the minimum displacement and the maximumdisplacement in the nth wave. In FIG. 3, the interval from when thedisplacement of the nth wave becomes minimum to when the displacement ofthe nth wave becomes minimum again is represented by Pn (for example, P₁for the first wave, P₂ for the second wave, and P₃ for the third wave).For calculating the logarithmic decrement, a difference between theminimum displacement and the maximum displacement that appear after thenth wave (A_(n+1) in the above equation, and A₄ in the displacement-timecurve shown in FIG. 3) is used, but the portion in which the pendulum107 stops (is adsorbed) after the maximum displacement is not used forcounting the number of waves. In addition, the portion in which thependulum 107 stops (is adsorbed) before the maximum displacement is notused for counting the number of waves. Accordingly, in thedisplacement-time curve shown in FIG. 3, the number of waves is 3 (n=3).

The aforementioned logarithmic decrement is considered as a value thatbecomes a parameter of the amount of a viscous component which isliberated from the surface of the magnetic layer in a case where thehead comes into contact with and slide on the surface of the magneticlayer and interposed between the surface of the magnetic layer and thehead. Presumably, in a case where the viscous component is attached toand accumulated on the head while the head is repeatedly sliding,spacing loss that causes the deterioration of electromagnetic conversioncharacteristics may occur. In contrast, the inventors consider that in acase where the logarithmic decrement of the magnetic layer surface inthe aforementioned magnetic recording medium is equal to or lower than0.050, that is, the state where the amount of the viscous component isreduced makes a contribution to the inhibition of the occurrence of thespacing loss by the attachment and accumulation of the viscous componenton the head. The inventors of the present invention assume that theinhibition of the occurrence of spacing loss may lead to the furtherinhibition of the deterioration of electromagnetic conversioncharacteristics that is caused during the repeated sliding due to theoccurrence of the spacing loss.

The details of the aforementioned viscous components are unclear. Theinventors of the present invention assume that the viscous component islikely to be derived from the resin used as a binder. Specifically, thebinder is as below. As a binder, as will be specifically describedlater, various resins can be used. The resin is a polymer (including ahomopolymer and a copolymer) of two or more kinds of polymerizablecompounds. Usually, the resin also includes a component having amolecular weight lower than the average molecular weight (hereinafter,described as “low-molecular weight binder component”). The inventors ofthe present invention consider that in a case where such a low-molecularweight binder component is liberated from the surface of the magneticlayer when the head slides on the surface of the magnetic layer andinterposed between the surface of the magnetic layer and the head, thespacing loss may occur. The low-molecular weight binder component isconsidered to have viscousness. The inventors of the present inventionassume that the logarithmic decrement determined by the pendulumviscoelasticity test may be a parameter of the amount of low-molecularweight binder component that is liberated from the surface of themagnetic layer when the head slides on the surface of the magneticlayer. In an aspect, the magnetic layer is formed by coating anon-magnetic support with a composition for forming a magnetic layercontaining a curing agent in addition to ferromagnetic hexagonal ferritepowder and a binder directly or through another layer and performing acuring treatment. By the curing treatment mentioned here, the binder andthe curing agent can perform a curing reaction (crosslinking reaction).Here, the inventors of the present invention consider that thelow-molecular weight binder component may exhibit poor reactivity in thecuring reaction although the reason is unclear. Therefore, the inventorsof the present invention assume that the property of the low-molecularweight binder component, in which the component does not easily stay inthe magnetic layer but is easily liberated from the magnetic layer, maybe one of the reasons why the low-molecular weight binder component isinterposed between the surface of the magnetic layer and the head whenthe head slides on the surface of the magnetic layer.

The points described so far are assumptions that the inventors of thepresent invention made regarding the mechanism which makes it possibleto inhibit the deterioration of the electromagnetic conversioncharacteristics in the magnetic recording medium even though the headrepeatedly slides on the surface of the magnetic layer. However, thepresent invention is not limited to the assumption. The presentspecification includes the assumption of the inventors of the presentinvention, and the present invention is not limited to the assumption.

Hereinbelow, various values will be more specifically described.

XRD Intensity Ratio

In the magnetic recording medium, the magnetic layer containsferromagnetic hexagonal ferrite powder. The XRD intensity ratio isdetermined by performing X-ray diffraction analysis on the magneticlayer containing the ferromagnetic hexagonal ferrite powder by using anIn-Plane method. Hereinafter, the X-ray diffraction analysis performedusing an In-Plane method will be described as “In-Plane XRD” as well.In-Plane XRD is performed by irradiating the surface of the magneticlayer with X-rays by using a thin film X-ray diffractometer under thefollowing conditions. Magnetic recording media are roughly classifiedinto a tape-like magnetic recording medium (magnetic tape) and adisc-like magnetic recording medium (magnetic disc). The magnetic tapeis measured in a longitudinal direction, and the magnetic disc ismeasured in a radius direction.

Radiation source used: Cu radiation (power of 45 kV, 200 mA)

Scan condition: 0.05 degree/step within a range of 20 to 40 degree, 0.1degree/min

Optical system used: parallel optical system

Measurement method: 2 θχ scan (X-ray incidence angle: 0.25°)

The above conditions are values set in the thin film X-raydiffractometer. As the thin film X-ray diffractometer, known instrumentscan be used. As one of the thin film X-ray diffractometers, SmartLabmanufactured by Rigaku Corporation can be exemplified. The sample usedfor In-Plane XRD analysis is not limited in terms of the size and shape,as long as it is a medium sample which is cut from a magnetic recordingmedium to be measured and enables the confirmation of a diffraction peakwhich will be described later.

Examples of the techniques of X-ray diffraction analysis include thinfilm X-ray diffraction and powder X-ray diffraction. By the powder X-raydiffraction, the X-ray diffraction of a powder sample is measured. Incontrast, by the thin film X-ray diffraction, it is possible to measurethe X-ray diffraction of a layer formed on a substrate and the like. Thethin film X-ray diffraction is classified into an In-Plane method and anOut-Of-Plane method. In the Out-Of-Plane method, the X-ray incidenceangle during measurement is within a range of 5.00° to 90.00°. Incontrast, in the In-Plane method, the X-ray incidence angle is generallywithin a range of 0.20° to 0.50°. In the present invention and thepresent specification, the X-ray incidence angle in In-Plane XRD is setto be 0.25° as described above. In the In-Plane method, the X-rayincidence angle is smaller than in the Out-Of-Plane method, and hencethe X-ray permeation depth is small. Accordingly, by the X-raydiffraction analysis (In-Plane XRD) using the In-Plane method, it ispossible to analyze the X-ray diffraction of a surface layer portion ofa sample to be measured. For the sample of the magnetic recordingmedium, the X-ray diffraction of the magnetic layer can be analyzed byIn-Plane XRD. In an X-ray diffraction spectrum obtained by theaforementioned In-Plane XRD, the aforementioned XRD intensity ratio isan intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110)of a diffraction peak of (110) plane of a crystal structure of thehexagonal ferrite to a peak intensity Int (114) of a diffraction peak of(114) plane of the crystal structure. Int is used as the abbreviation ofintensity. In the X-ray diffraction spectrum obtained by In-Plane XRD(ordinate: intensity, abscissa: diffraction angle 2 θχ (degree)), thediffraction peak of (114) plane is a peak detected at 2 θχ that iswithin a range of 33 to 36 degree, and the diffraction peak of (110)plane is a peak detected at 2 θχ that is within a range of 29 to 32degree.

Among diffraction planes, (114) plane of the crystal structure of thehexagonal ferrite is positioned close to a direction of a magnetizationeasy axis (c-axis direction) of the particles of the ferromagnetichexagonal ferrite powder (hexagonal ferrite particles). The (110) planeof the hexagonal ferrite crystal structure is positioned in a directionorthogonal the direction of the magnetization easy axis.

Regarding the aforementioned former particles among the hexagonalferrite particles contained in the magnetic layer, the inventors of thepresent invention considered that the more the direction of theparticles orthogonal to the magnetization easy axis is parallel to thesurface of the magnetic layer, the more difficult it is for the abrasiveto permeate the inside of the magnetic layer by being supported by thehexagonal ferrite particles. In contrast, regarding the former particlesin the magnetic layer, the inventors of the present invention considerthat the more the direction of the particles orthogonal to themagnetization easy axis is perpendicular to the surface of the magneticlayer, the easier it is for the abrasive to permeate the inside of themagnetic layer because it is difficult for the abrasive to be supportedby the hexagonal ferrite powder. Furthermore, the inventors of thepresent invention assume that in the X-ray diffraction spectradetermined by In-Plane XRD, in a case where the intensity ratio (Int(110)/Int (114); XRD intensity ratio) of the peak intensity Int (110) ofthe diffraction peak of (110) plane to the peak intensity Int (114) ofthe diffraction peak of (114) plane of the hexagonal ferrite crystalstructure is high, it means that the magnetic layer contains a largeamount of the former particles whose direction orthogonal to thedirection of the magnetization easy axis is more parallel to the surfaceof the magnetic layer; and in a case where the XRD intensity ratio islow, it means that the magnetic layer contains a small amount of suchformer particles. In addition, the inventors consider that in a casewhere the XRD intensity ratio is equal to or lower than 4.0, it meansthat the former particles, that is, the particles, which support theabrasive pushed into the inside of the magnetic layer and exert aninfluence on the degree of the permeation of the abrasive, merelysupport the abrasive, and as a result, the abrasive can appropriatelypermeate the inside of the magnetic layer at the time when a head slideson the surface of the magnetic layer. The inventors of the presentinvention assume that the aforementioned mechanism may make acontribution to hinder the occurrence of the head scraping even thoughthe head repeatedly slides on the surface of the magnetic layer. Incontrast, the inventors of the present invention consider that the statein which the abrasive appropriately protrudes from the surface of themagnetic layer when the head slides on the surface of the magnetic layermay make a contribution to the reduction of the contact area (realcontact) between the surface of the magnetic layer and the head. Theinventors consider that the larger the real contact area, the strongerthe force applied to the surface of the magnetic layer from the headwhen the head slides on the surface of the magnetic layer, and as aresult, the surface of the magnetic layer is damaged and scraped.Regarding this point, the inventors of the present invention assume thatin a case where the XRD intensity ratio is equal to or higher than 0.5,it shows that the aforementioned former particles are present in themagnetic layer in a state of being able to support the abrasive withallowing the abrasive to appropriately protrude from the surface of themagnetic layer when the head slides on the surface of the magneticlayer.

From the viewpoint of further inhibiting the deterioration of theelectromagnetic conversion characteristics, the XRD intensity ratio ispreferably equal to or lower than 3.5, and more preferably equal to orlower than 3.0. From the same viewpoint, the XRD intensity ratio ispreferably equal to or higher than 0.7, and more preferably equal to orhigher than 1.0. The XRD intensity ratio can be controlled by thetreatment conditions of the alignment treatment performed in themanufacturing process of the magnetic recording medium. As the alignmenttreatment, it is preferable to perform a vertical alignment treatment.The vertical alignment treatment can be preferably performed by applyinga magnetic field in a direction perpendicular to a surface of the wet(undried) coating layer of the composition for forming a magnetic layer.The further the alignment conditions are strengthened, the higher theXRD intensity ratio tends to be. Examples of the treatment conditions ofthe alignment treatment include the magnetic field intensity in thealignment treatment and the like. The treatment conditions of thealignment treatment are not particularly limited, and may be set suchthat an XRD intensity ratio of equal to or higher than 0.5 and equal toor lower than 4.0 can be achieved. For example, the magnetic fieldintensity in the vertical alignment treatment can be set to be 0.10 to0.80 T or 0.10 to 0.60 T. As the dispersibility of the ferromagnetichexagonal ferrite powder in the composition for forming a magnetic layeris improved, the value of the XRD intensity ratio tends to increase bythe vertical alignment treatment.

Squareness Ratio in Vertical Direction

The squareness ratio in a vertical direction is a squareness ratiomeasured in a vertical direction of the magnetic recording medium.“Vertical direction” described regarding the squareness ratio refers toa direction orthogonal to the surface of the magnetic layer. Forexample, in a case where the magnetic recording medium is a tape-likemagnetic recording medium, that is, a magnetic tape, the verticaldirection is a direction orthogonal to a longitudinal direction of themagnetic tape. The squareness ratio in a vertical direction is measuredusing a vibrating sample fluxmeter. Specifically, in the presentinvention and the present specification, the squareness ratio in avertical direction is a value determined by carrying out scanning in thevibrating sample fluxmeter by applying a maximum external magnetic fieldof 1,194 kA/m (15 kOe) as an external magnetic field to the magneticrecording medium, at a measurement temperature of 23° C.±1° C. under thecondition of a scan rate of 4.8 kA/m/sec (60 Oe/sec), which is usedafter being corrected for a demagnetizing field. The measured squarenessratio is a value from which the magnetization of a sample probe of thevibrating sample fluxmeter is subtracted as background noise.

The squareness ratio in a vertical direction of the magnetic recordingmedium is equal to or higher than 0.65. The inventors of the presentinvention assume that the squareness ratio in a vertical direction ofthe magnetic recording medium can be a parameter of the amount of theaforementioned latter particles (fine particles) present in the magneticlayer that are considered to induce the reduction in the hardness of themagnetic layer. It is considered that the magnetic layer in the magneticrecording medium having a squareness ratio in a vertical direction ofequal to or higher than 0.65 has high hardness because of containing asmall amount of such fine particles and is hardly scraped by the slidingof the head on the surface of the magnetic layer. Presumably, becausethe surface of the magnetic layer is hardly scraped, it is possible toinhibit the electromagnetic conversion characteristics fromdeteriorating due to the occurrence of spacing loss resulting fromforeign substances that occur due to the scraping of the surface of themagnetic layer. From the viewpoint of further inhibiting thedeterioration of the electromagnetic conversion characteristics, thesquareness ratio in a vertical direction is preferably equal to orhigher than 0.68, more preferably equal to or higher than 0.70, evenmore preferably equal to or higher than 0.73, and still more preferablyequal to or higher than 0.75. In principle, the squareness ratio is 1.00at most. Accordingly, the squareness ratio in a vertical direction ofthe magnetic recording medium is equal to or lower than 1.00. Thesquareness ratio in a vertical direction may be equal to or lower than0.95, 0.90, 0.87, or 0.85, for example. The larger the value of thesquareness ratio in a vertical direction, the smaller the amount of theaforementioned fine latter particles in the magnetic layer. Therefore,it is considered that from the viewpoint of the hardness of the magneticlayer, the value of the squareness ratio is preferably large.Accordingly, the squareness ratio in a vertical direction may be higherthan the upper limit exemplified above.

The inventors of the present invention consider that in order to obtaina squareness ratio in a vertical direction of equal to or higher than0.65, it is preferable to inhibit fine particles from occurring due topartial chipping of particles in the step of preparing the compositionfor forming a magnetic layer. Specific means for inhibiting theoccurrence of chipping will be described later.

Logarithmic Decrement

The logarithmic decrement obtained by performing a pendulumviscoelasticity test on the surface of the magnetic layer of themagnetic recording medium is equal to or lower than 0.050. Thelogarithmic decrement of equal to or lower than 0.050 can make acontribution to the inhibition of the deterioration of electromagneticconversion characteristics during the repeated sliding. From theviewpoint of further inhibiting the deterioration of electromagneticconversion characteristics during the repeated sliding, the logarithmicdecrement is preferably equal to or lower than 0.048, more preferablyequal to or lower than 0.045, and even more preferably equal to or lowerthan 0.040. In contrast, from the viewpoint of inhibiting thedeterioration of electromagnetic conversion characteristics during therepeated sliding, the lower the logarithmic decrement, the morepreferable. Therefore, the lower limit is not particularly limited. Forexample, the logarithmic decrement can be equal to or higher than 0.010or equal to or higher than 0.015. Here, the logarithmic decrement may belower than the value exemplified above. Specific aspects of the meansfor adjusting the logarithmic decrement will be described later.

Hereinafter, the magnetic recording medium will be more specificallydescribed.

Magnetic Layer

Ferromagnetic Hexagonal Ferrite Powder

The magnetic layer of the magnetic recording medium containsferromagnetic hexagonal ferrite powder as ferromagnetic powder.Regarding the ferromagnetic hexagonal ferrite powder, a magnetoplumbitetype (referred to as “M type” as well), a W type, a Y type, and Z typeare known as crystal structures of the hexagonal ferrite. Theferromagnetic hexagonal ferrite powder contained in the magnetic layermay take any of the above crystal structures. The crystal structures ofthe hexagonal ferrite contain an iron atom and a divalent metal atom asconstituent atoms. The divalent metal atom is a metal atom which canbecome a divalent cation as an ion, and examples thereof include alkaliearth metal atoms such as a barium atom, a strontium atom, and a calciumatom, a lead atom, and the like. For example, the hexagonal ferritecontaining a barium atom as a divalent metal atom is barium ferrite, andthe hexagonal ferrite containing a strontium atom is strontium ferrite.The hexagonal ferrite may be a mixed crystal of two or more kinds ofhexagonal ferrite. As one of the mixed crystals, a mixed crystal ofbarium ferrite and strontium ferrite can be exemplified.

As the parameter of a particle size of the ferromagnetic hexagonalferrite powder, activation volume can be used. “Activation volume” isthe unit of magnetization inversion. The activation volume described inthe present invention and the present specification is a value measuredusing a vibrating sample fluxmeter in an environment with an atmospherictemperature of 23° C.±1° C. by setting a magnetic field sweep rate to be3 minutes and 30 minutes for a coercive force Hc measurement portion,and determined from the following relational expression of Hc and anactivation volume V.Hc=2Ku/Ms{1[(kT/KuV)ln(At/0.693)]^(1/2)}

[In the expression, Ku: anisotropy constant, Ms: saturationmagnetization, k: Boltzmann constant, T: absolute temperature, V:activation volume, A: spin precession frequency, t: magnetic fieldinversion time]

Examples of methods for achieving the high-density recording include amethod of increasing a filling rate of ferromagnetic powder in themagnetic layer by reducing the particle size of the ferromagnetic powdercontained in the magnetic layer. In this respect, the activation volumeof the ferromagnetic hexagonal ferrite powder is preferably equal to orless than 2,500 nm³, more preferably equal to or less than 2,300 nm³,and even more preferably equal to or less than 2,000 nm³. In contrast,from the viewpoint of the stability of magnetization, the activationvolume is preferably equal to or greater than 800 nm³, more preferablyequal to or greater than 1,000 nm³, and even more preferably equal to orgreater than 1,200 nm³, for example.

In order to identify the shape of the particles constituting theferromagnetic hexagonal ferrite powder, the ferromagnetic hexagonalferrite powder is imaged using a transmission electron microscope at a100,000× magnification, and the image is printed on photographic papersuch that the total magnification thereof becomes 500,000×. In the imageof the particles obtained in this way, the outlines of particles(primary particles) are traced using a digitizer so as to identify theparticle shape. The primary particles refer to independent particles notbeing aggregated with each other. The particles are imaged using atransmission electron microscope at an acceleration voltage of 300 kV byusing a direct method. For performing observation and measurement usingthe transmission electron microscope, for example, it is possible to usea transmission electron microscope H-9000 manufactured by HitachiHigh-Technologies Corporation and image analysis software KS-400manufactured by Carl Zeiss A G. Regarding the shape of the particlesconstituting the ferromagnetic hexagonal ferrite powder, “plate-like”means a shape having two plate surfaces facing each other. Amongparticle shapes that do not have such plate surfaces, a shape having amajor axis and a minor axis different from each other is “elliptical”.The major axis is an axis (straight line) which is the longest diameterof a particle. The minor axis is a straight line which is the longestdiameter of a particle in a direction orthogonal to the major axis. Ashape in which the major axis and the minor axis are the same as eachother, that is, a shape in which the major axis length equals the minoraxis length is “spherical”. A shape in which the major axis and theminor axis cannot be identified is called “amorphous”. The imagingperformed for identifying the particle shape by using a transmissionelectron microscope is carried out without performing an alignmenttreatment on the powder to be imaged. The ferromagnetic hexagonalferrite powder used for preparing the composition for forming a magneticlayer and the ferromagnetic hexagonal ferrite powder contained in themagnetic layer may take any of the plate-like shape, the ellipticalshape, the spherical shape and the amorphous shape.

The mean particle size relating to various powders described in thepresent invention and the present specification is an arithmetic mean ofsizes determined for 500 particles randomly extracted using a particleimage captured as described above. The mean particle size shown inexamples which will be described later is a value obtained using atransmission electron microscope H-9000 manufactured by HitachiHigh-Technologies Corporation as a transmission electron microscope andimage analysis software KS-400 manufactured by Carl Zeiss A G as imageanalysis software.

For details of the ferromagnetic hexagonal ferrite powder, for example,paragraphs “0134” to “0136” in JP2011-216149A can also be referred to.

The content (filling rate) of the ferromagnetic hexagonal ferrite powderin the magnetic layer is preferably within a range of 50% to 90% bymass, and more preferably within a range of 60% to 90% by mass. Themagnetic layer contains at least a binder and an abrasive as componentsother than the ferromagnetic hexagonal ferrite powder, and canoptionally contain one or more kinds of additives. From the viewpoint ofimproving the recording density, the filling rate of the ferromagnetichexagonal ferrite powder in the magnetic layer is preferably high.

Binder and Curing Agent

The magnetic layer of the magnetic recording medium contains a binder.As the binder, one or more kinds of resins are used. The resin may be ahomopolymer or a copolymer. As the binder contained in the magneticlayer, a binder selected from an acryl resin obtained by copolymerizinga polyurethane resin, a polyester resin, a polyamide resin, a vinylchloride resin, styrene, acrylonitrile, or methyl methacrylate, acellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin,a polyvinyl alkyral resin such as polyvinyl acetal or polyvinyl butyralcan be used singly, or a plurality of resins can be used by being mixedtogether. Among these, a polyurethane resin, an acryl resin, a celluloseresin, and a vinyl chloride resin are preferable. These resins can beused as a binder in a non-magnetic layer and/or a back coating layerwhich will be described later. Regarding the aforementioned binders,paragraphs “0029” to “0031” in JP2010-24113A can be referred to. Theaverage molecular weight of the resin used as a binder can be equal toor greater than 10,000 and equal to or less than 200,000 in terms of aweight-average molecular weight, for example. The weight-averagemolecular weight in the present invention and the present specificationis a value determined by measuring a molecular weight by gel permeationchromatography (GPC) and expressing the molecular weight in terms ofpolystyrene. As the measurement conditions, the following conditions canbe exemplified. The weight-average molecular weight shown in exampleswhich will be described later is a value determined by measuring amolecular weight under the following measurement conditions andexpressing the molecular weight in terms of polystyrene.

GPC instrument: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (Inner Diameter)×30.0 cm)

Eluent: tetrahydrofuran (THF)

At the time of forming the magnetic layer, it is possible to use acuring agent together with a resin usable as the aforementioned binder.In an aspect, the curing agent can be a thermosetting compound which isa compound experiencing a curing reaction (crosslinking reaction) byheating. In another aspect, the curing agent can be a photocurablecompound experiencing a curing reaction (crosslinking reaction) by lightirradiation. The curing agent experiences a curing reaction in themanufacturing process of the magnetic recording medium. In this way, atleast a portion of the curing agent can be contained in the magneticlayer, in a state of reacting (cross-linked) with other components suchas the binder. The curing agent is preferably a thermosetting compoundwhich is suitably polyisocyanate. For details of polyisocyanate,paragraphs “0124” and “0125” in JP2011-216149A can be referred to. Thecuring agent can be used by being added to the composition for forming amagnetic layer, in an amount of 0 to 80.0 parts by mass with respect to100.0 parts by mass of the binder and preferably in an amount of 50.0 to80.0 parts by mass from the viewpoint of improving the hardness of themagnetic layer.

Abrasive

The magnetic layer of the magnetic recording medium contains anabrasive. The abrasive refers to non-magnetic powder having a Mohshardness of higher than 8, and is preferably non-magnetic powder havinga Mohs hardness of equal to or higher than 9. The abrasive may be powderof an inorganic substance (inorganic powder) or powder of an organicsubstance (organic powder), and is preferably inorganic powder. Theabrasive is preferably inorganic powder having a Mohs hardness of higherthan 8, and even more preferably inorganic powder having Mohs hardnessof equal to or higher than 9. The maximum value of the Mohs hardness is10 which is the Mohs hardness of diamond. Specific examples of theabrasive include powder of alumina (Al₂O₃), silicon carbide, boroncarbide (B₄C), TiC, cerium oxide, zirconium oxide (ZrO₂), diamond, andthe like. Among these, alumina powder is preferable. Regarding thealumina powder, paragraph “0021” in JP2013-229090A can also be referredto. As a parameter of the particle size of the abrasive, specificsurface area can be used. The larger the specific surface area, thesmaller the particle size. It is preferable to use an abrasive having aspecific surface area (hereinafter, described as “BET specific surfacearea”) of equal to or greater than 14 m²/g, which is measured forprimary particles by a Brunauer-Emmett-Teller (BET) method. From theviewpoint of dispersibility, it is preferable to use an abrasive havinga BET specific surface area of equal to or less than 40 m²/g. Thecontent of the abrasive in the magnetic layer is preferably 1.0 to 20.0parts by mass with respect to 100.0 parts by mass of the ferromagnetichexagonal ferrite powder.

Additive

The magnetic layer contains the ferromagnetic hexagonal ferrite powder,the binder, and the abrasive, and may further contain one or more kindsof additives if necessary. As one of the additives, the aforementionedcuring agent can be exemplified. Examples of the additives that can becontained in the magnetic layer include non-magnetic powder, alubricant, a dispersant, a dispersion aid, a fungicide, an antistaticagent, an antioxidant, and the like. As one of the additives which canbe used in the magnetic layer containing the abrasive, the dispersantdescribed in paragraphs “0012” to “0022” in JP2013-131285A can beexemplified as a dispersant for improving the dispersibility of theabrasive in the composition for forming a magnetic layer.

Examples of the dispersant also include known dispersants such as acarboxy group-containing compound and a nitrogen-containing compound.The nitrogen-containing compound may be any one of a primary aminerepresented by NH₂R, a secondary amine represented by NHR₂, and atertiary amine represented by NR₃, for example. R represents anystructure constituting the nitrogen-containing compound, and a pluralityof R's present in the compound may be the same as or different from eachother. The nitrogen-containing compound may be a compound (polymer)having a plurality of repeating structures in a molecule. The inventorsof the present invention consider that because the nitrogen-containingportion of the nitrogen-containing compound functions as a portionadsorbed onto the surface of particles of the ferromagnetic hexagonalferrite powder, the nitrogen-containing compound can act as adispersant. Examples of the carboxy group-containing compound includefatty acids such as oleic acid. Regarding the carboxy group-containingcompound, the inventors of the present invention consider that becausethe carboxy group functions as a portion adsorbed onto the surface ofparticles of the ferromagnetic hexagonal ferrite powder, the carboxygroup-containing compound can act as a dispersant. It is also preferableto use the carboxy group-containing compound and the nitrogen-containingcompound in combination.

Examples of the non-magnetic powder that can be contained in themagnetic layer include non-magnetic powder (hereinafter, described as“projection-forming agent” as well) which can contribute to the controlof frictional characteristics by forming projections on the surface ofthe magnetic layer. As such a non-magnetic powder, it is possible to usevarious non-magnetic powders generally used in a magnetic layer. Thenon-magnetic powder may be inorganic powder or organic powder. In anaspect, from the viewpoint of uniformizing the frictionalcharacteristics, it is preferable that the particle size distribution ofthe non-magnetic powder is not polydisperse distribution having aplurality of peaks in the distribution but monodisperse distributionshowing a single peak. From the viewpoint of ease of availability of themonodisperse particles, the non-magnetic powder is preferably inorganicpowder. Examples of the inorganic powder include powder of a metaloxide, a metal carbonate, a metal sulfate, a metal nitride, a metalcarbide, a metal sulfide, and the like. The particles constituting thenon-magnetic powder are preferably colloidal particles, and morepreferably colloidal particles of an inorganic oxide. From the viewpointof ease of availability of the monodisperse particles, the inorganicoxide constituting the colloidal particles of an inorganic oxide ispreferably silicon dioxide (silica). The colloidal particles of aninorganic oxide are preferably colloidal silica (colloidal silicaparticles). In the present invention and the present specification,“colloidal particles” refer to the particles which can form a colloidaldispersion by being dispersed without being precipitated in a case wherethe particles are added in an amount of 1 g per 100 mL of at least oneorganic solvent among methyl ethyl ketone, cyclohexanone, toluene, ethylacetate, and a mixed solvent containing two or more kinds of thesolvents described above at any mixing ratio. In another aspect, thenon-magnetic powder is also preferably carbon black. The mean particlesize of the non-magnetic powder is 30 to 300 nm for example, andpreferably 40 to 200 nm. The content of the non-magnetic powder in themagnetic layer is, with respect to 100.0 parts by mass of theferromagnetic hexagonal ferrite powder, preferably 1.0 to 4.0 parts bymass and more preferably 1.5 to 3.5 parts by mass, because then thenon-magnetic filler can demonstrate better the function thereof.

As various additives that can be optionally contained in the magneticlayer, commercially available products or those manufactured by knownmethods can be selected and used according to the desired properties.

The magnetic layer described so far can be provided on the surface ofthe non-magnetic support, directly or indirectly through a non-magneticlayer.

Non-Magnetic Layer

Next, a non-magnetic layer will be described.

The magnetic recording medium may have the magnetic layer directly onthe surface of the non-magnetic support, or may have a non-magneticlayer containing non-magnetic powder and a binder between thenon-magnetic support and the magnetic layer. The non-magnetic powdercontained in the non-magnetic layer may be inorganic powder or organicpowder. Furthermore, carbon black or the like can also be used. Examplesof the inorganic powder include powder of a metal, a metal oxide, ametal carbonate, a metal sulfate, a metal nitride, a metal carbide, ametal sulfide, and the like. These non-magnetic powders can be obtainedas commercially available products, or can be manufactured by knownmethods. For details of the non-magnetic powder, paragraphs “0036” to“0039” in JP2010-24113A can be referred to. The content (filling rate)of the non-magnetic powder in the non-magnetic layer is preferablywithin a range of 50% to 90% by mass, and more preferably within a rangeof 60% to 90% by mass.

For other details of the binder, the additives, and the like of thenon-magnetic layer, known techniques relating to the non-magnetic layercan be applied. For example, regarding the type and content of thebinder, the type and content of the additives, and the like, knowntechniques relating to the magnetic layer can also be applied.

In the present invention and the present specification, the non-magneticlayer also includes a substantially non-magnetic layer which containsnon-magnetic powder with a small amount of ferromagnetic powder as animpurity or by intention, for example. Herein, the substantiallynon-magnetic layer refers to a layer having a remnant flux density ofequal to or lower than 10 mT or a coercive force of equal to or lowerthan 7.96 kA/m (100 Oe) or having a remnant flux density of equal to orlower than 10 mT and a coercive force of equal to or lower than 7.96kA/m (100 Oe). It is preferable that the non-magnetic layer does nothave remnant flux density and coercive force.

Non-Magnetic Support

Next, a non-magnetic support (hereinafter, simply described as “support”as well) will be described. Examples of the non-magnetic support includeknown supports such as biaxially oriented polyethylene terephthalate,polyethylene naphthalate, polyamide, polyamide imide, and aromaticpolyamide. Among these, polyethylene terephthalate, polyethylenenaphthalate, and polyamide are preferable. These supports may besubjected to corona discharge, a plasma treatment, an easy adhesiontreatment, a heat treatment, and the like in advance.

Back Coating Layer

The magnetic recording medium can have a back coating layer containingnon-magnetic powder and a binder, on a surface side of the non-magneticsupport opposite to a surface side provided with the magnetic layer. Itis preferable that the back coating layer contains either or both ofcarbon black and inorganic powder. Regarding the binder contained in theback coating layer and various additives which can be optionallycontained therein, known techniques relating to the back coating layercan be applied, and known techniques relating to the formulation of themagnetic layer and/or the non-magnetic layer can also be applied.

Various Thicknesses

The thickness of the non-magnetic support and each layer in the magneticrecording medium will be described below.

The thickness of the non-magnetic support is 3.0 to 80.0 μm for example,preferably 3.0 to 50.0 μm, and more preferably 3.0 to 10.0 μm.

The thickness of the magnetic layer can be optimized according to thesaturation magnetization of the magnetic head to be used, the length ofhead gap, the band of recording signals, and the like. The thickness ofthe magnetic layer is generally 10 nm to 100 nm. From the viewpoint ofhigh-density recording, the thickness of the magnetic layer ispreferably 20 to 90 nm, and more preferably 30 to 70 nm. The magneticlayer may be constituted with at least one layer, and may be separatedinto two or more layers having different magnetic characteristics.Furthermore, the constitution relating to known multi-layered magneticlayers can be applied. In a case where the magnetic layer is separatedinto two or more layers, the thickness of the magnetic layer means thetotal thickness of the layers.

The thickness of the non-magnetic layer is equal to or greater than 50nm for example, preferably equal to or greater than 70 nm, and morepreferably equal to or greater than 100 nm. In contrast, the thicknessof the non-magnetic layer is preferably equal to or less than 800 nm,and more preferably equal to or less than 500 nm.

The thickness of the back coating layer is preferably equal to or lessthan 0.9 μm, and more preferably 0.1 to 0.7 μm.

The thickness of each layer and the non-magnetic support of the magneticrecording medium can be measured by known film thickness measurementmethods. For example, a cross section of the magnetic recording mediumin a thickness direction is exposed by known means such as ion beams ora microtome, and then the exposed cross section is observed using ascanning electron microscope. By observing the cross section, athickness of one site in the thickness direction or an arithmetic meanof thicknesses of two or more randomly extracted sites, for example, twosites can be determined as various thicknesses. Furthermore, as thethickness of each layer, a design thickness calculated from themanufacturing condition may be used.

Manufacturing Process

Preparation of Composition for Forming Each Layer

The step of preparing a composition for forming the magnetic layer, thenon-magnetic layer, or the back coating layer generally includes atleast a kneading step, a dispersion step, and a mixing step that isperformed if necessary before and after the aforementioned steps. Eachof the aforementioned steps may be divided into two or more stages. Thecomponents used for preparing the composition for forming each layer maybe added at the initial stage or in the middle of any of the abovesteps. As a solvent, it is possible to use one kind of solvent or two ormore kinds of solvents generally used for manufacturing a coating-typemagnetic recording medium. Regarding the solvent, for example, paragraph“0153” in JP2011-216149A can be referred to. Furthermore, each of thecomponents may be added in divided portions in two or more steps. Forexample, the binder may be added in divided portions in the kneadingstep, the dispersion step, and the mixing step performed afterdispersion to adjust viscosity. In order to manufacture theaforementioned magnetic recording medium, the manufacturing techniquesknown in the related art can be used in various steps. In the kneadingstep, it is preferable to use an instrument having strong kneadingforce, such as an open kneader, a continuous kneader, a pressurizedkneader, or an extruder. For details of the kneading treatment,JP1989-106338A (JP-H01-106338A) and JP1989-79274A (JP-H01-79274A) can bereferred to. As a disperser, known ones can be used. The composition forforming each layer may be filtered by a known method before beingsubjected to a coating step. The filtration can be performed using afilter, for example. As the filter used for the filtration, for example,it is possible to use a filter having a pore size of 0.01 to 3 μm (forexample, a filter made of glass fiber, a filter made of polypropylene,or the like).

Regarding the dispersion treatment for the composition for forming amagnetic layer, as described above, it is preferable to inhibit theoccurrence of chipping. In order to inhibit chipping, in the step ofpreparing the composition for forming a magnetic layer, it is preferableto perform the dispersion treatment for the ferromagnetic hexagonalferrite powder in two stages, such that coarse aggregates of theferromagnetic hexagonal ferrite powder are disintegrated in the firststage of the dispersion treatment and then the second stage of thedispersion treatment is performed in which the collision energy appliedto the particles of the ferromagnetic hexagonal ferrite powder due tothe collision with dispersion beads is smaller than in the firstdispersion treatment. According to the dispersion treatment describedabove, it is possible to achieve both of the improvement ofdispersibility of the ferromagnetic hexagonal ferrite powder and theinhibition of occurrence of chipping.

Examples of preferred aspects of the aforementioned two-stage dispersiontreatment include a dispersion treatment including a first stage ofobtaining a dispersion liquid by performing a dispersion treatment onthe ferromagnetic hexagonal ferrite powder, the binder, and the solventin the presence of first dispersion beads, and a second stage ofperforming a dispersion treatment on the dispersion liquid obtained bythe first stage in the presence of second dispersion beads having a beadsize and a density smaller than a bead size and a density of the firstdispersion beads. Hereinafter, the dispersion treatment of theaforementioned preferred aspect will be further described.

In order to improve the dispersibility of the ferromagnetic hexagonalferrite powder, it is preferable that the first and second stagesdescribed above are performed as a dispersion treatment preceding themixing of the ferromagnetic hexagonal ferrite powder with other powdercomponents. For example, in a case where the magnetic layer containingthe abrasive and the aforementioned non-magnetic powder is formed, it ispreferable to perform the aforementioned first and second stages as adispersion treatment for a liquid (magnetic liquid) containing theferromagnetic hexagonal ferrite powder, the binder, the solvent, andadditives optionally added, before the abrasive and the non-magneticpowder are mixed with the liquid.

The bead size of the second dispersion beads is preferably equal to orless than 1/100 and more preferably equal to or less than 1/500 of thebead size of the first dispersion beads. Furthermore, the bead size ofthe second dispersion beads can be, for example, equal to or greaterthan 1/10,000 of the bead size of the first dispersion beads, but is notlimited to this range. For example, the bead size of the seconddispersion beads is preferably within a range of 80 to 1,000 nm. Incontrast, the bead size of the first dispersion beads can be within arange of 0.2 to 1.0 mm, for example.

In the present invention and the present specification, the bead size isa value measured by the same method as used for measuring theaforementioned mean particle size of powder.

The second stage described above is preferably performed under thecondition in which the second dispersion beads are present in an amountequal to or greater than 10 times the amount of the ferromagnetichexagonal ferrite powder, and more preferably performed under thecondition in which the second dispersion beads are present in an amountthat is 10 to 30 times the amount of the ferromagnetic hexagonal ferritepowder, based on mass.

The amount of the first dispersion beads in the first stage ispreferably within the above range.

The second dispersion beads are beads having a density smaller than thatof the first dispersion beads. “Density” is obtained by dividing mass(unit: g) of the dispersion beads by volume (unit: cm³) thereof. Thedensity is measured by the Archimedean method. The density of the seconddispersion beads is preferably equal to or lower than 3.7 g/cm³, andmore preferably equal to or lower than 3.5 g/cm³. The density of thesecond dispersion beads may be equal to or higher than 2.0 g/cm³ forexample, and may be lower than 2.0 g/cm³. In view of density, examplesof the second dispersion beads preferably include diamond beads, siliconcarbide beads, silicon nitride beads, and the like. In view of densityand hardness, examples of the second dispersion beads preferably includediamond beads.

The first dispersion beads are preferably dispersion beads having adensity of higher than 3.7 g/cm³, more preferably dispersion beadshaving a density of equal to or higher than 3.8 g/cm³, and even morepreferably dispersion beads having a density of equal to or higher than4.0 g/cm³. The density of the first dispersion beads may be equal to orlower than 7.0 g/cm³ for example, and may be higher than 7.0 g/cm³. Asthe first dispersion beads, zirconia beads, alumina beads, or the likeare preferably used, and zirconia beads are more preferably used.

The dispersion time is not particularly limited and may be set accordingto the type of the disperser used and the like.

Coating Step, Cooling Step, Heating and Drying Step, BurnishingTreatment Step, and Curing Step

The magnetic layer can be formed by directly coating the non-magneticsupport with the composition for forming a magnetic layer or byperforming multilayer coating by sequentially or simultaneously applyingthe composition for forming a non-magnetic layer. For details of coatingfor forming each layer, paragraph “0066” in JP2010-231843A can bereferred to.

In a preferred aspect, the magnetic layer can be formed through amagnetic layer-forming step including a coating step of coating anon-magnetic support with a composition for forming a magnetic layercontaining ferromagnetic hexagonal ferrite powder, a binder, anabrasive, a curing agent, and a solvent directly or through anon-magnetic layer so as to form a coating layer, a heating and dryingstep of drying the coating layer by a heating treatment, and a curingstep of performing a curing treatment on the coating layer. It ispreferable that the magnetic layer-forming step includes a cooling stepof cooling the coating layer between the coating step and the heatingand drying step and further includes a burnishing treatment step ofperforming a burnishing treatment on a surface of the coating layerbetween the heating and drying step and the curing step.

It is considered that performing the cooling step and the burnishingtreatment step in the aforementioned magnetic layer-forming step ispreferable means for making the logarithmic decrement equal to or lowerthan 0.050. Specifically, the reason is as below.

Presumably, performing the cooling step of cooling the coating layerbetween the coating step and the heating and drying step may make acontribution to the localization of the aforementioned viscous componentwithin the surface of the coating layer and/or a surface layer portionin the vicinity of the surface. It is considered that this is because ina case where the coating layer of the composition for forming a magneticlayer is cooled before the heating and drying step, the viscouscomponent may easily move to the surface of the coating layer and/or thesurface layer portion when the solvent is volatilized during the heatingand drying step. Here, the reason is unclear. Furthermore, it isconsidered that in a case where the burnishing treatment is performed onthe surface of the coating layer in which the viscous component islocalized within the surface thereof and/or the surface layer portion,the viscous component can be removed. Presumably, in a case where thecuring step is performed after the viscous component is removed in thisway, the logarithmic decrement may become equal to or lower than 0.050.Here, this is merely a presumption, and the present invention is notlimited thereto.

As described above, the composition for forming a magnetic layer can beused for multilayer coating by being sequentially and simultaneouslyused with the composition for forming a non-magnetic layer. In apreferred aspect, the magnetic recording medium can be manufactured bysequential multilayer coating. The manufacturing process including thesequential multilayer coating preferably can be performed as below. Thenon-magnetic layer is formed through a coating step of coating anon-magnetic support with the composition for forming a non-magneticlayer so as to form a coating layer and a heating and drying step ofdrying the formed coating layer by performing a heating treatment. Then,the magnetic layer is formed through a coating step of coating theformed non-magnetic layer with the composition for forming a magneticlayer so as to form a coating layer and a heating and drying step ofdrying the formed coating layer by performing a heating treatment.

Hereinafter, a specific aspect of the aforementioned manufacturingmethod will be described based on FIG. 4, but the present invention isnot limited to the following specific aspect.

FIG. 4 is a schematic process chart showing a specific aspect of stepsfor manufacturing a magnetic recording medium which has a non-magneticlayer and a magnetic layer in this order on one surface of anon-magnetic support and has a back coating layer on the other surfaceof the non-magnetic support. In the aspect shown in FIG. 4, an operationof feeding a non-magnetic support (long film) from a feeding portion andwinding up the non-magnetic support in a winding-up portion iscontinuously performed, various treatments such as coating, drying, andalignment are performed in each portion or each zone shown in FIG. 4,and in this way, a non-magnetic layer and a magnetic layer can be formedon one surface of the running non-magnetic support by sequentialmultilayer coating and a back coating layer can be formed on the othersurface thereof. This manufacturing method is the same as themanufacturing method that is usually performed for manufacturing acoating-type magnetic recording medium, except for the manufacturingmethod mentioned herein includes a cooling zone in the magneticlayer-forming step and includes the burnishing treatment step before thecuring treatment.

In a first coating portion, the non-magnetic support fed from thefeeding portion is coated with the composition for forming anon-magnetic layer (coating step of the composition for forming anon-magnetic layer).

After the coating step, in a first heating treatment zone, the coatinglayer of the composition for forming a non-magnetic layer formed by thecoating step is heated, thereby drying the coating layer (heating anddrying step). The heating and drying step can be performed by causingthe non-magnetic support, which has the coating layer of the compositionfor forming a non-magnetic layer, to pass through a heating environment.The atmospheric temperature of the heating environment can be about 60°C. to 140° C., for example. The atmospheric temperature is not limitedto the above range, as long as the temperature enables the coating layerto be dried by causing the volatilization of the solvent. Furthermore,optionally, a heated gas may be blown to the surface of the coatinglayer. The point described so far is true for the heating and dryingstep in a second heating treatment zone and the heating and drying stepin a third heating treatment zone which will be described later.

Then, in a second coating portion, the non-magnetic layer formed by theheating and drying step performed in the first heating treatment zone iscoated with the composition for forming a magnetic layer (coating stepof the composition for forming a magnetic layer).

After the coating step, in a cooling zone, the coating layer of thecomposition for forming a magnetic layer formed by the coating step iscooled (cooling step). For example, the cooling step can be performed bycausing the non-magnetic support, in which the aforementioned coatinglayer is formed on the non-magnetic layer, to pass through a coolingenvironment. The atmospheric temperature of the cooling environment canbe preferably within a range of −10° C. to 0° C., and more preferablywithin a range of −5° C. to 0° C. The time taken for performing thecooling step (for example, the time from when any portion of the coatinglayer comes into the cooling zone to when the portion comes out of thecooling zone (hereinafter, referred to as “staying time” as well)) isnot particularly limited. The longer the staying time is, the lower thelogarithmic decrement tends to be. Therefore, it is preferable to adjustthe staying time by performing a preliminary experiment as necessarysuch that a logarithmic decrement of equal to or lower than 0.050 can berealized. In the cooling step, a cooled gas may be blown to the surfaceof the coating layer.

Thereafter, in an aspect in which an alignment treatment is performed,while the coating layer of the composition for forming a magnetic layeris remaining wet, an alignment treatment is performed on theferromagnetic hexagonal ferrite powder in the coating layer in analignment zone. Regarding the alignment treatment, it is possible toapply various known techniques including those described in paragraph“0067” in JP2010-231843A without any limitation. As described above,from the viewpoint of controlling the XRD intensity ratio, it ispreferable to perform a vertical alignment treatment as the alignmenttreatment. Regarding the alignment treatment, the above description canalso be referred to.

The coating layer having undergone the alignment treatment is subjectedto the heating and drying step in the second heating treatment zone.

Then, in a third coating portion, a surface, which is opposite to thesurface on which the non-magnetic layer and the magnetic layer areformed, of the non-magnetic support is coated with a composition forforming a back coating layer, thereby forming a coating layer (coatingstep of the composition for forming a back coating layer). Thereafter,in the third heating treatment zone, the coating layer is dried byperforming a heating treatment.

In this way, it is possible to obtain a magnetic recording medium inwhich the heated and dried coating layer of the composition for forminga magnetic layer is provided on the non-magnetic layer on one surface ofthe non-magnetic support and the back coating layer is provided on theother surface of the non-magnetic support. The magnetic recording mediumobtained herein will be subjected to various treatments, which will bedescribed later, and become a magnetic recording medium as a product.

The obtained magnetic recording medium is wound up in a winding-upportion and then cut (slit) in the size of the magnetic recording mediumas a product. Slitting can be performed using a known cutting machine.

Before the slit magnetic recording medium is subjected to a curingtreatment (heating, light irradiation, or the like) according to thetype of the curing agent contained in the composition for forming amagnetic layer, the surface of the heated and dried coating layer of thecomposition for forming a magnetic layer is subjected to the burnishingtreatment (burnishing treatment step between the heating and drying stepand the curing step). By the burnishing treatment, it is possible toremove the viscous component that has been cooled in the cooling zoneand has moved to the surface of the coating layer and/or the surfacelayer portion. The inventors of the present invention assume that theremoval of the viscous component makes the logarithmic decrement becomeequal to or lower than 0.050. However, as described above, this ismerely an assumption, and the present invention is not limited thereto.

The burnishing treatment is a treatment of rubbing the surface of atreatment target with a member (for example, a polishing tape or agrinding tool such as a grinding blade or a grinding foil), and can beperformed in the same manner as that adopted for performing a knownburnishing treatment for manufacturing a coating-type magnetic recordingmedium. Here, in the related art, after the cooling step and the heatingand drying step, the burnishing treatment is not performed in the stagebefore the curing step. By performing the burnishing treatment in theaforementioned stage, the logarithmic decrement can become equal to orlower than 0.050, and this is a point that the inventors of the presentinvention have newly discovered.

The burnishing treatment can be performed preferably by performingeither or both of rubbing (polishing) the surface of the coating layeras a treatment target with a polishing tape and rubbing (grinding) thesurface of the coating layer as a treatment target with a grinding tool.In a case where the composition for forming a magnetic layer contains anabrasive, it is preferable to use a polishing tape containing at leastone kind of abrasive having Mohs hardness higher than that of theabrasive contained in the composition. As the polishing tape,commercially available products may be used, or polishing tapes preparedby known methods may be used. Furthermore, as the grinding tool, it ispossible to use a fixed blade, a diamond foil, a known grinding bladesuch as a rotary blade, a grinding foil, or the like. In addition, awiping treatment may be performed in which the surface of the coatinglayer rubbed with the polishing tape and/or the grinding tool is wipedby a wiping material. For details of preferred polishing tape, grindingtool, burnishing treatment, and wiping treatment, paragraphs “0034” to“0048”, FIG. 1, and examples in JP1994-52544A (JP-H06-52544A) can bereferred to. The further the burnishing treatment is intensified, thelower the value of logarithmic decrement tends to be. As the hardness ofthe abrasive used as an abrasive contained in the polishing tape isincreased and as the amount of the abrasive in the polishing tape isincreased, the burnishing treatment can be further intensified. Inaddition, as the hardness of the grinding tool used as a grinding toolis increased, the burnishing treatment can be further intensified.Regarding the conditions of the burnishing treatment, as the slidingspeed of the member (for example, the polishing tape or the grindingtool) sliding on the surface of the coating layer as a treatment targetis increased, the burnishing treatment can be further intensified. Thesliding speed can be increased by increasing either or both of the speedat which the member moves and the speed at which the magnetic tape as atreatment target moves.

After the burnishing treatment (burnishing treatment step), a curingtreatment is performed on the coating layer of the composition forforming a magnetic layer. In the aspect shown in FIG. 4, the coatinglayer of the composition for forming a magnetic layer is subjected to asurface smoothing treatment before the curing treatment is performedafter the burnishing treatment. The surface smoothing treatment ispreferably performed by a calender treatment. For details of thecalender treatment, for example, paragraph “0026” in JP2010-231843A canbe referred to. The further the calender treatment is intensified, thefurther the surface of the magnetic tape can be smoothened. The calendertreatment can be further intensified by either or both of increasing thesurface temperature of a calender roll (calender temperature) andincreasing a calender pressure.

Then, a curing treatment is performed on the coating layer of thecomposition for forming a magnetic layer according to the type of thecuring agent contained in the coating layer (curing step). The curingtreatment can be performed by a treatment such as a heating treatment orlight irradiation according to the type of the curing agent contained inthe coating layer. The conditions of the curing treatment are notparticularly limited, and may be appropriately set according to theformulation of the composition for forming a magnetic layer used forforming the coating layer, the type of the curing agent, the thicknessof the coating layer, and the like. For example, in a case where thecoating layer is formed using a composition for forming a magnetic layercontaining polyisocyanate as a curing agent, a heating treatment ispreferred as the curing treatment. In a case where the curing agent iscontained in a layer other than the magnetic layer, the curing reactionof such a layer can proceed by the curing treatment mentioned herein.Furthermore, a curing step may be additionally performed. After thecuring step, the burnishing treatment may be additionally performed.

By the method described so far, a magnetic recording medium according toan aspect of the present invention can be obtained. Here, theaforementioned manufacturing method is merely an example. The value ofeach of the XRD intensity ratio, the squareness ratio in a verticaldirection, and the logarithmic decrement of the magnetic layer can becontrolled within the aforementioned range by any means that can adjustthe value, and this aspect is also included in the present invention.

The aforementioned magnetic recording medium according to an aspect ofthe present invention can be a tape-like magnetic recording medium(magnetic tape), for example. Generally, the magnetic tape isdistributed and used in a state of being accommodated in a magnetic tapecartridge. In the magnetic tape, in order to enable head tracking servoto be performed in a drive, a servo pattern can also be formed by aknown method. By mounting the magnetic tape cartridge on a drive(referred to as “magnetic tape device” as well) and running the magnetictape in the drive such that a magnetic head contacts and slides on asurface of the magnetic tape (surface of a magnetic layer), informationis recorded on the magnetic tape and reproduced. In order tocontinuously or intermittently perform repeated reproduction of theinformation recorded on the magnetic tape, the magnetic tape is causedto repeatedly run in the drive. According to an aspect of the presentinvention, it is possible to provide a magnetic tape in which theelectromagnetic conversion characteristics thereof hardly deteriorateeven though the head repeatedly slides on the surface of the magneticlayer while the tape is repeatedly running. Here, the magnetic recordingmedium according to an aspect of the present invention is not limited tothe magnetic tape. The magnetic recording medium according to an aspectof the present invention is suitable as various magnetic recording media(a magnetic tape, a disc-like magnetic recording medium (magnetic disc),and the like) used in a sliding-type magnetic recording and/orreproduction device. The sliding-type device refers to a device in whicha head contacts and slides on a surface of a magnetic layer in a casewhere information is recorded on a magnetic recording medium and/or therecorded information is reproduced. Such a device includes at least amagnetic tape and one or more magnetic heads for recording and/orreproducing information.

In the aforementioned sliding-type device, as the running speed of themagnetic tape is increased, it is possible to shorten the time taken forrecording information and reproducing the recorded information. Therunning speed of the magnetic tape refers to a relative speed of themagnetic tape and the magnetic head. Generally, the running speed is setin a control portion of the device. As the running speed of the magnetictape is increased, the pressure increases which is applied to both thesurface of the magnetic layer and the magnetic head in a case where thesurface of the magnetic layer and the magnetic head come into contactwith each other. As a result, either or both of head scraping andmagnetic layer scraping tend to easily occur. Accordingly, it isconsidered that the higher the running speed, the easier it is for theelectromagnetic conversion characteristics to deteriorate during therepeated sliding. In the field of magnetic recording, the improvement ofrecording density is required. However, as the recording density isincreased, the influence of the signal interference between the adjacentheads becomes stronger, and hence the electromagnetic conversioncharacteristics tend to be more easily deteriorate when the spacing lossis increased due to the repeated sliding. As described so far, as therunning speed and the recording density are increased further, thedeterioration of the electromagnetic conversion characteristics duringthe repeated sliding tends to be more apparent. In contrast, even inthis case, according to the magnetic recording medium of an aspect ofthe present invention, it is possible to inhibit the deterioration ofthe electromagnetic conversion characteristics during the repeatedsliding. The magnetic tape according to an aspect of the presentinvention is suitable for being used in a sliding-type device in whichthe running speed of the magnetic tape is, for example, equal to orhigher than 5 m/sec (for example, 5 to 20 m/sec). In addition, themagnetic tape according to an aspect of the present invention issuitable as a magnetic tape for recording and reproducing information ata line recording density of equal to or higher than 250 kfci, forexample. The unit kfci is the unit of a line recording density (thisunit cannot be expressed in terms of the SI unit system). The linerecording density can be equal to or higher than 250 kfci or equal to orhigher than 300 kfci, for example. Furthermore, the line recordingdensity can be equal to or lower than 800 kfci or higher than 800 kfci,for example.

EXAMPLES

Hereinafter, the present invention will be described based on examples,but the present invention is not limited to the aspects shown in theexamples. In the following description, unless otherwise specified,“part” and “%” represent “part by mass” and “% by mass” respectively.Furthermore, unless otherwise specified, the steps and the evaluationsdescribed below were performed in an environment with an atmospherictemperature of 23° C.±1° C.

Example 1

The formulations of compositions for forming each layer will be shownbelow.

Formulation of composition for forming magnetic layer

-   -   Magnetic liquid    -   Plate-like ferromagnetic hexagonal ferrite powder (M-type barium        ferrite): 100.0 parts    -   (activation volume: 1,500 nm³)    -   Oleic acid: 2.0 parts    -   Vinyl chloride copolymer (MR-104 manufactured by ZEON        CORPORATION): 10.0 parts    -   SO₃Na group-containing polyurethane resin: 4.0 parts    -   (weight-average molecular weight: 70,000, SO₃Na group: 0.07        meq/g)    -   Amine-based polymer (DISPERBYK-102 manufactured by BYK-Chemie        GmbH): 6.0 parts    -   Methyl ethyl ketone: 150.0 parts    -   Cyclohexanone: 150.0 parts    -   Abrasive liquid    -   α-Alumina: 6.0 parts    -   (BET specific surface area: 19 m²/g, Mohs hardness: 9)    -   SO₃Na group-containing polyurethane resin: 0.6 parts    -   (weight-average molecular weight: 70,000, SO₃Na group: 0.1        meq/g)    -   2,3-Dihydroxynaphthalene: 0.6 parts    -   Cyclohexanone: 23.0 parts    -   Projection-forming agent liquid    -   Colloidal silica: 2.0 parts        -   (mean particle size: 80 nm)    -   Methyl ethyl ketone: 8.0 parts    -   Lubricant and curing agent liquid    -   Stearic acid: 3.0 parts    -   Amide stearate: 0.3 parts    -   Butyl stearate: 6.0 parts    -   Methyl ethyl ketone: 110.0 parts    -   Cyclohexanone: 110.0 parts    -   Polyisocyanate (CORONATE (registered trademark) L manufactured        by Tosoh Corporation): 3.0 parts

Formulation of composition for forming non-magnetic layer

-   -   Non-magnetic inorganic powder α iron oxide: 100.0 parts        -   (mean particle size: 10 nm, BET specific surface area: 75            m²/g)    -   Carbon black: 25.0 parts        -   (mean particle size: 20 nm)    -   SO₃Na group-containing polyurethane resin: 18.0 parts        -   (weight-average molecular weight: 70,000, content of SO₃Na            group: 0.2 meq/g)    -   Stearic acid: 1.0 part    -   Cyclohexanone: 300.0 parts    -   Methyl ethyl ketone: 300.0 parts

Formulation of composition for forming back coating layer

-   -   Non-magnetic inorganic powder α iron oxide: 80.0 parts        -   (mean particle size: 0.15 μm, BET specific surface area: 52            m²/g)    -   Carbon black: 20.0 parts        -   (mean particle size: 20 nm)    -   Vinyl chloride copolymer: 13.0 parts    -   Sulfonate group-containing polyurethane resin: 6.0 parts    -   Phenyl phosphonate: 3.0 parts    -   Cyclohexanone: 155.0 parts    -   Methyl ethyl ketone: 155.0 parts    -   Stearic acid: 3.0 parts    -   Butyl stearate: 3.0 parts    -   Polyisocyanate: 5.0 parts    -   Cyclohexanone: 200.0 parts

Preparation of Composition for Forming Magnetic Layer

The composition for forming a magnetic layer was prepared by thefollowing method.

The aforementioned various components of a magnetic liquid weredispersed for 24 hours by a batch-type vertical sand mill by usingzirconia beads (first dispersion beads, density: 6.0 g/cm³) having abead size of 0.5 mm (first stage) and then filtered using a filterhaving a pore size of 0.5 μm, thereby preparing a dispersion liquid A.The amount of the used zirconia beads was 10 times the mass of theferromagnetic hexagonal ferrite powder based on mass.

Then, the dispersion liquid A was dispersed for the time shown in Table1 by a batch-type vertical sand mill by using diamond beads (seconddispersion beads, density: 3.5 g/cm³) having a bead size shown in Table1 (second stage), and the diamond beads were separated using acentrifuge, thereby preparing a dispersion liquid (dispersion liquid B).The following magnetic liquid is the dispersion liquid B obtained inthis way.

The aforementioned various components of an abrasive liquid were mixedtogether and put into a horizontal beads mill disperser together withzirconia beads having a bead size of 0.3 mm, and the volume thereof wasadjusted such that bead volume/(volume of abrasive liquid)+ bead volumeequaled 80%. The mixture was subjected to a dispersion treatment byusing the beads mill for 120 minutes, and the liquid formed after thetreatment was taken out and subjected to ultrasonic dispersion andfiltration treatment by using a flow-type ultrasonic dispersion andfiltration device. In this way, an abrasive liquid was prepared.

The prepared magnetic liquid and abrasive liquid as well as theremaining components were introduced into a dissolver stirrer, stirredfor 30 minutes at a circumferential speed of 10 m/sec, and then treatedin 3 passes with a flow-type ultrasonic disperser at a flow rate of 7.5kg/min. Thereafter, the resultant was filtered through a filter having apore size of 1 μm, thereby preparing a composition for forming amagnetic layer.

The activation volume of the ferromagnetic hexagonal ferrite powderdescribed above is a value measured and calculated using the powder thatwas in the same powder lot as the ferromagnetic hexagonal ferrite powderused for preparing the composition for forming a magnetic layer. Theactivation volume was measured using a vibrating sample fluxmeter(manufactured by TOEI INDUSTRY, CO., LTD.) by setting a magnetic fieldsweep rate to be 3 minutes and 30 minutes for a coercive force Hcmeasurement portion, and calculated from the relational expressiondescribed above. The activation volume was measured in an environmentwith a temperature of 23° C.±1° C.

Preparation of Composition for Forming Non-Magnetic Layer

The aforementioned various components of a composition for forming anon-magnetic layer were dispersed by a batch-type vertical sand mill for24 hours by using zirconia beads having a bead size of 0.1 mm and thenfiltered using a filter having a pore size of 0.5 μm, thereby preparinga composition for forming a non-magnetic layer.

Preparation of Composition for Forming Back Coating Layer

Among the aforementioned various components of a composition for forminga back coating layer, the components except for the lubricant (stearicacid and butyl stearate), polyisocyanate, and 200.0 parts ofcyclohexanone were kneaded and diluted using an open kneader and thensubjected to a dispersion treatment in 12 passes by a horizontal beadsmill disperser by using zirconia beads having a bead size of 1 mm bysetting a bead filling rate to be 80% by volume, a circumferential speedof the rotor tip to be 10 m/sec, and a retention time per pass to be 2minutes. Then, other components described above were added thereto,followed by stirring with a dissolver. The obtained dispersion liquidwas filtered using a filter having a pore size of 1 μm, therebypreparing a composition for forming a back coating layer.

Preparation of Magnetic Tape

A magnetic tape was prepared according to the specific aspect shown inFIG. 4. Specifically, the magnetic tape was prepared as below.

A support made of polyethylene naphthalate having a thickness of 5.0 μmwas fed from a feeding portion. In the first coating portion, onesurface of the support was coated with the composition for forming anon-magnetic layer such that the thickness thereof became 100 nm afterdrying, the composition was dried in the first heating treatment zone(atmospheric temperature: 100° C.), thereby forming a coating layer.

Then, in the second coating portion, the non-magnetic layer was coatedwith the composition for forming a magnetic layer such that thethickness thereof became 70 nm after drying, thereby forming a coatinglayer. While the formed coating layer is remaining wet, the cooling stepwas performed by causing the support to pass through the cooling zoneadjusted to have an atmospheric temperature of 0° C. for the stayingtime shown in Table 1. Thereafter, in the alignment zone, the verticalalignment treatment was performed by applying a magnetic field having anintensity shown in Table 1 thereto in a direction perpendicular to thesurface of the coating layer, and then the coating layer was dried inthe second heating treatment zone (atmospheric temperature: 100° C.).

Subsequently, in the third coating portion, a surface, which wasopposite to the surface on which the non-magnetic layer and the magneticlayer were formed, of the support made of polyethylene naphthalate wascoated with the composition for forming a back coating layer such thatthe thickness thereof became 0.4 μm after drying, thereby forming acoating layer. The formed coating layer was dried in the third heatingtreatment zone (atmospheric temperature: 100° C.).

The magnetic tape obtained in this way was slit in a width of ½ inches(0.0127 meters), and then the burnishing treatment and the wipingtreatment were performed on the surface of the coating layer of thecomposition for forming a magnetic layer. The burnishing treatment andthe wiping treatment were performed using a treatment device constitutedas described in FIG. 1 in JP1994-52544A (JP-H06-52544A), in which acommercially available polishing tape (manufactured by FUJIFILMCorporation, trade name: MA22000, abrasive: diamond/Cr₂O₃/colcothar) wasused as a polishing tape, a commercially available sapphire blade(manufactured by KYOCERA Corporation, width: 5 mm, length: 35 mm, tipangle: 60°) was used as a grinding blade, and a commercially availablewiping material (manufactured by KURARAY CO., LTD., trade name: WRP736)was used as a wiping material. As the treatment conditions, thetreatment conditions in Example 12 in JP1994-52544A (JP-H06-52544A) wereadopted.

After the aforementioned burnishing treatment and the wiping treatment,by using a calender rolls constituted solely with metal rolls, acalender treatment (surface smoothing treatment) was performed at aspeed of 80 m/min, a line pressure of 300 kg/cm (294 kN/m), and acalender temperature (calender roll surface temperature) of 90° C.

Subsequently, the tape was subjected to a heating treatment (curingtreatment) for 36 hours in an environment with an atmospherictemperature of 70° C., and then a servo pattern was formed on themagnetic layer by using a commercially available servowriter.

In this way, a magnetic tape of Example 1 was obtained.

Evaluation of Deterioration of Electromagnetic ConversionCharacteristics (Signal-to-Noise-Ratio; SNR)

The electromagnetic conversion characteristics of the magnetic tape ofExample 1 were measured using a ½-inch (0.0127 meters) reel tester, towhich a head was fixed, by the following method.

The running speed of the magnetic tape (relative speed of magnetichead/magnetic tape) was set to be the value shown in Table 1. AMetal-In-Gap (MIG) head (gap length: 0.15 μm, track width: 1.0 μm) wasused as a recording head, and as a recording current, a recordingcurrent optimal for each magnetic tape was set. As a reproducing head, aGiant-Magnetoresistive (GMR) head having an element thickness of 15 nm,a shield gap of 0.1 μm, and a lead width of 0.5 μm was used. Signalswere recorded at a line recording density shown in Table 1, and thereproduced signals were measured using a spectrum analyzer manufacturedby ShibaSoku Co., Ltd. A ratio between an output value of carriersignals and integrated noise in the entire bandwidth of the spectrum wastaken as SNR. For measuring SNR, the signals of a portion of themagnetic tape, in which signals were sufficiently stabilized afterrunning, were used.

Under the above conditions, each magnetic tape was caused to performreciprocating running in 5,000 passes at 1,000 m/l pass in anenvironment with a temperature of 40° C. and a relative humidity of 80%,and then SNR was measured. Then, a difference between SNR of the 1^(st)pass and SNR of the 5,000^(th) pass (SNR of the 5,000^(th) pass-SNR ofthe 1^(st) pass) was calculated.

The recording and reproduction described above were performed by causingthe head to slide on a surface of the magnetic layer of the magnetictape.

Examples 2 to 17

Magnetic tapes were prepared in the same manner as in Example 1 exceptthat various items shown in Table 1 were changed as shown in Table 1,and the deterioration of the electromagnetic conversion characteristics(SNR) of the prepared magnetic tapes was evaluated.

In Table 1, in the example for which “N/A” is described in the column ofDispersion beads and the column of Time, the composition for forming amagnetic layer was prepared without performing the second stage in thedispersion treatment for the magnetic liquid.

In Table 1, in the example for which “N/A” is described in the column ofMagnetic field intensity for vertical alignment treatment, the magneticlayer was formed without performing the alignment treatment.

In Table 1, in the example for which “not performed” is described in thecolumn of Staying time in cooling zone and in the column of Burnishingtreatment before curing treatment, the magnetic tape was prepared by amanufacturing process in which a cooling zone is not included in themagnetic layer-forming step and the burnishing treatment as well as thewiping treatment were not performed before the curing treatment.

A portion of each of the prepared magnetic tapes was used for theevaluation of the deterioration of electromagnetic conversioncharacteristics (SNR), and the other portion thereof was used forphysical property evaluation described below.

Evaluation of Physical Properties of Magnetic Tape

(1) XRD Intensity Ratio

From each of the magnetic tapes of Examples 1 to 17, tape samples werecut.

By using a thin film X-ray diffractometer (SmartLab manufactured byRigaku Corporation), X-rays were caused to enter a surface of themagnetic layer of the cut tape sample, and In-Plane XRD was performed bythe method described above.

From the X-ray diffraction spectrum obtained by In-Plane XRD, a peakintensity Int (114) of a diffraction peak of (114) plane and a peakintensity Int (110) of a diffraction peak of (110) plane of thehexagonal ferrite crystal structure were determined, and the XRDintensity ratio (Int (110)/Int (114)) was calculated.

(2) Squareness Ratio in Vertical Direction

For each of the magnetic tapes of Examples 1 to 17, by using a vibratingsample fluxmeter (manufactured by TOEI INDUSTRY, CO., LTD.), asquareness ratio in a vertical direction was determined by the methoddescribed above.

(3) Measurement of Logarithmic Decrement of Magnetic Layer Surface

The logarithmic decrement of the magnetic layer surface of the magnetictape was acquired by the method described above by using a rigid-bodypendulum type physical properties testing instrument RPT-3000 Wmanufactured by A&D Company, Limited (pendulum: rigid-body pendulumFRB-100 manufactured by A&D Company, weight: not employed, round-bartype cylinder edge: RBP-040 manufactured by A&D Company, substrate:glass substrate, a rate of temperature increase of substrate: 5° C./min)as the measurement device.

A commercially available slide glass was cut into a size of 25 mm (shortside)×50 mm (long side) and employed as the glass substrate. In a statewhere the magnetic tape was placed on the center part of the glasssubstrate so that the longitudinal direction of the magnetic tape wasparallel to the direction of the short side of the glass substrate, fourcorners of the magnetic tape were fixed on the glass substrate with afixing tape (Kapton tape manufactured by Du Pont-Toray Co., Ltd.). Then,portions of the magnetic tape protruding from the glass substrate werecut out. In the above manner, the measurement sample was placed on aglass substrate by being fixed at 4 portions as shown in FIG. 1. Theadsorption time was set to be 1 second, the measurement interval was setto be 7 to 10 seconds, and a displacement-time curve was created for the86^(th) measurement interval. By using the curve, the logarithmicdecrement was determined. The measurement was performed in anenvironment with a relative humidity of 50%.

The results obtained as above are shown in Table 1.

TABLE 1 Dispersion treatment for magnetic liquid Second stage Dispersionbeads Formulation amount (mass of beads with respect to Magnetic fieldBurnishing mass of ferromagnetic hexagonal intensity for verticalStaying time in treatment before Type Bead size ferrite powder) Timealignment treatment cooling zone curing treatment Example 1 Diamond 500nm 10 times greater 1 h 0.15 T 1 second Performed Example 2 Diamond 500nm 10 times greater 1 h 0.20 T 1 second Performed Example 3 Diamond 500nm 10 times greater 1 h 0.30 T 1 second Performed Example 4 Diamond 500nm 10 times greater 1 h 0.50 T 1 second Performed Example 5 Diamond 500nm 20 times greater 1 h 0.15 T 1 second Performed Example 6 Diamond 500nm 10 times greater 1 h 0.30 T 1 second Performed Example 7 Diamond 500nm 10 times greater 1 h 0.30 T 60 seconds Performed Example 8 Diamond500 nm 10 times greater 1 h 0.30 T 180 seconds Performed Example 9 N/AN/A N/A N/A N/A Not performed Not performed Example 10 N/A N/A N/A N/AN/A Not performed Not performed Example 11 N/A N/A N/A N/A N/A Notperformed Not performed Example 12 Diamond 500 nm 10 times greater 1 h0.15 T Not performed Not performed Example 13 N/A N/A N/A N/A N/A 1second Performed Example 14 N/A N/A N/A N/A 0.15 T 1 second PerformedExample 15 N/A N/A N/A N/A 0.30 T 1 second Performed Example 16 Diamond500 nm 10 times greater 1 h 1.00 T 1 second Performed Example 17 Diamond500 nm 10 times greater 1 h N/A 1 second Performed Logarithmic decrementof magnetic layer XRD intensity ratio Squareness ratio in Running speedLine recording SNR surface Int (110)/Int (114) vertical direction ofmagnetic tape density deterioration Example 1 0.048 0.5 0.70 6 m/s 270kfci −0.5 dB Example 2 0.048 1.5 0.75 6 m/s 270 kfci −0.9 dB Example 30.048 2.3 0.80 6 m/s 270 kfci −0.7 dB Example 4 0.048 4.0 0.85 6 m/s 270kfci −0.5 dB Example 5 0.048 0.7 0.83 6 m/s 270 kfci −0.4 dB Example 60.048 2.3 0.80 8 m/s 300 kfci −0.8 dB Example 7 0.033 2.3 0.80 8 m/s 300kfci −0.6 dB Example 8 0.015 2.3 0.80 8 m/s 300 kfci −0.3 dB Example 90.060 0.2 0.55 4 m/s 200 kfci −1.0 dB Example 10 0.060 0.2 0.55 6 m/s270 kfci −3.0 dB Example 11 0.060 0.2 0.55 8 m/s 300 kfci −4.3 dBExample 12 0.060 0.5 0.70 6 m/s 270 kfci −2.5 dB Example 13 0.048 0.20.55 6 m/s 270 kfci −2.3 dB Example 14 0.048 3.8 0.63 6 m/s 270 kfci−2.5 dB Example 15 0.048 5.0 0.75 6 m/s 270 kfci −2.5 dB Example 160.048 6.1 0.90 6 m/s 270 kfci −2.8 dB Example 17 0.048 0.3 0.66 6 m/s270 kfci −2.5 dB

From the results shown in Table 1, it was confirmed that in Examples 1to 8, in which each of the XRD intensity ratio, the squareness ratio ina vertical direction, and the logarithmic decrement of the magneticlayer surface of the magnetic tape is within the range described above,the electromagnetic conversion characteristics hardly deteriorate eventhough reproduction is repeated by causing the head to slide on thesurface of the magnetic layer, unlike in Examples 9 to 17.

In Examples 9 to 11, magnetic tapes of the same physical properties wereused, but the magnetic tapes had different running speeds and differentline recording densities. Through the comparison between Examples 9 to11, it is possible to confirm that as the running speed or the linerecording density of the magnetic tape is increased, the deteriorationof the electromagnetic conversion characteristics during the repeatedsliding becomes more apparent. In Examples 1 to 8, the deterioration ofthe electromagnetic conversion characteristics during the repeatedsliding could be inhibited.

One aspect of the present invention can be useful in the technical fieldof magnetic recording media for data storage such as data backup tapes.

What is claimed is:
 1. A magnetic recording medium comprising: anon-magnetic support; and a magnetic layer which is provided on thesupport and contains ferromagnetic powder and a binder, wherein theferromagnetic powder is ferromagnetic hexagonal ferrite powder, themagnetic layer contains an abrasive, an intensity ratio (Int (110)/Int(114)) of a peak intensity Int (110) of a diffraction peak of (110)plane of a crystal structure of the hexagonal ferrite, determined byperforming X-ray diffraction analysis on the magnetic layer by using anIn-Plane method, to a peak intensity Int (114) of a diffraction peak of(114) plane of the crystal structure is equal to or higher than 0.5 andequal to or lower than 4.0, a squareness ratio of the magnetic recordingmedium in a vertical direction is equal to or higher than 0.65 and equalto or lower than 1.00, and a logarithmic decrement obtained byperforming a pendulum viscoelasticity test on a surface of the magneticlayer is equal to or lower than 0.050.
 2. The magnetic recording mediumaccording to claim 1, wherein the squareness ratio in a verticaldirection is equal to or higher than 0.65 and equal to or lower than0.90.
 3. The magnetic recording medium according to claim 1, wherein thelogarithmic decrement is equal to or higher than 0.010 and equal to orlower than 0.050, and the logarithmic decrement is determined by thefollowing method; securing a measurement sample of the magnetic tapewith the measurement surface, which is the surface on the magnetic layerside, facing upward on a substrate in a pendulum viscoelasticity tester;disposing a columnar cylinder edge which is 4 mm in diameter andequipped with a pendulum 13 g in weight on the measurement surface ofthe measurement sample such that the long axis direction of the columnarcylinder edge runs parallel to the longitudinal direction of themeasurement sample; raising the surface temperature of the substrate onwhich the measurement sample has been positioned at a rate of less thanor equal to 5° C./min up to 80° C.; inducing initial oscillation of thependulum; monitoring the displacement of the pendulum while it isoscillating to obtain a displacement-time curve for a measurementinterval of greater than or equal to 10 minutes; and obtaining thelogarithmic decrement Δ from the following equation:$\Delta = \frac{{\ln\left( \frac{A_{1}}{A_{2}} \right)} + {\ln\left( \frac{A_{2}}{A_{3}} \right)} + {\ldots\mspace{14mu}{\ln\left( \frac{A_{n}}{A_{n + 1}} \right)}}}{n}$wherein the interval from one minimum displacement to the next minimumdisplacement is adopted as one wave period; the number of wavescontained in the displacement-time curve during one measurement intervalis denoted by n, the difference between the minimum displacement and themaximum displacement of the n^(th) wave is denoted by An, and thelogarithmic decrement is calculated using the difference between thenext minimum displacement and maximum displacement of the n^(th) wave(A_(n+1) in the above equation).
 4. The magnetic recording mediumaccording to claim 2, wherein the logarithmic decrement is equal to orhigher than 0.010 and equal to or lower than 0.050, and the logarithmicdecrement is determined by the following method; securing a measurementsample of the magnetic tape with the measurement surface, which is thesurface on the magnetic layer side, facing upward on a substrate in apendulum viscoelasticity tester; disposing a columnar cylinder edgewhich is 4 mm in diameter and equipped with a pendulum 13 g in weight onthe measurement surface of the measurement sample such that the longaxis direction of the columnar cylinder edge runs parallel to thelongitudinal direction of the measurement sample; raising the surfacetemperature of the substrate on which the measurement sample has beenpositioned at a rate of less than or equal to 5° C./min up to 80° C.;inducing initial oscillation of the pendulum; monitoring thedisplacement of the pendulum while it is oscillating to obtain adisplacement-time curve for a measurement interval of greater than orequal to 10 minutes; and obtaining the logarithmic decrement Δ from thefollowing equation:$\Delta = \frac{{\ln\left( \frac{A_{1}}{A_{2}} \right)} + {\ln\left( \frac{A_{2}}{A_{3}} \right)} + {\ldots\mspace{14mu}{\ln\left( \frac{A_{n}}{A_{n + 1}} \right)}}}{n}$wherein the interval from one minimum displacement to the next minimumdisplacement is adopted as one wave period; the number of wavescontained in the displacement-time curve during one measurement intervalis denoted by n, the difference between the minimum displacement and themaximum displacement of the n^(th) wave is denoted by An, and thelogarithmic decrement is calculated using the difference between thenext minimum displacement and maximum displacement of the n^(th) wave(A_(n+1) in the above equation).
 5. The magnetic recording mediumaccording to claim 1, further comprising: a non-magnetic layercontaining non-magnetic powder and a binder between the non-magneticsupport and the magnetic layer.
 6. The magnetic recording mediumaccording to claim 1, further comprising: a back coating layercontaining non-magnetic powder and a binder on a surface, which isopposite to a surface provided with the magnetic layer, of thenon-magnetic support.
 7. The magnetic recording medium according toclaim 1, which is a magnetic tape.